Kelvin-Helmholtz contraction

The contraction of a ball of gas under gravity,
accompanied by the radiation of the lost potential
energy as heat. This was proposed by William
Thomson (Lord Kelvin) and Heinrich Helmholtz
as the most efficient means by which the Sun could remain hot for a long
period. Unfortunately, it isn't long enough. The so-called Kelvin-Helmholtz
timescale of the Sun is only 20 to 30 million years. Although stars
are now known to shine by nuclear fusion,
rather than by gravitational collapse, Kelvin-Helmholtz contraction is still
believed to be a valid description of the way infant stars behave during
a portion of their pre-main-sequence evolution.